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EARLIER THIS YEAR a 22-year-old college dropout, Jared Lee Loughner, shot Arizona congresswoman Gabrielle Giffords through the head near a Tucson supermarket, causing significant damage to Giffords’s brain. In the same shooting spree, Loughner killed or wounded 18 others, including a federal judge and a nine-year-old girl.

Information from Loughner’s postings on YouTube and elsewhere online suggests that he is severely mentally ill. Individuals with serious mental illnesses have perpetrated other recent shoot-ings, including the massacre in 2007 at ­Virginia Tech in which a college senior, ­Seung-Hui Cho, killed 32 people and wounded 17. These events and the accompanying media coverage have probably fed the public’s perception that most profoundly mentally ill people are violent. Surveys show that 60 to 80 percent of the public believes that those diagnosed with schizophrenia, in particular, are likely to commit violent acts.

Although studies have pointed to a slight increase in the risk of violent behaviors among those afflicted with major psychiatric ailments, a closer examination of the research suggests that these disorders are not strong predictors of aggressive behavior. In reality, severely mentally ill people account for only 3 to 5 percent of violent crimes in the general population. The data indicate that other behaviors are likely to be better harbingers of physical aggression—an insight that may help us prevent outbursts of rage in the future.

A Tenuous Tie
Not all psychological and emotional disorders portend violence, even in society’s eyes. In this column, we refer only to severe mental illness—meaning schizophrenia, bipolar disorder or psychotic depression. Symptoms of schizophrenia include marked disturbances in thoughts, emotions and behaviors; delusions (fixed false beliefs); hallucinations (perceiving things that are not physically present); disorganization; and withdrawal from social activities. Bipolar disorder is usually characterized by swings between depression and mania, which involves euphoria and grandiosity, a boost in energy and less need for sleep. Psychotic depression includes acute depressive symptoms, along with delusions or hallucinations, or both.

Most researchers investigating the question of aggression in the mentally ill have found a small but telling association between violence and significant psychological disturbance. In a 2009 meta-analysis, or quantitative review, of 204 studies exploring this connection, psychologist Kevin S. Douglas of Simon Fraser University and his associates found a slightly greater likelihood of aggressive behaviors among those with severe mental illnesses.

Yet this connection is much weaker than the public seems to believe it is and does not necessarily mean that these serious disorders cause violence. The causation could be in the reverse direction: engaging in chronic aggression (stemming from some other source) may create stress that triggers the illness in those predisposed to it. Alternatively, a third factor could spawn both a psychiatric condition and violence.

Rather than thinking of people with severe mental illness as generally dangerous, scientists are now pinpointing those other factors that might augur violent behavior more reliably. One strong candidate is drug abuse. Revealing results from the MacArthur Violence Risk Assessment Study in 1998, sociologist Henry J. Steadman of Policy Research Associates and his colleagues reported that almost a third of severely mentally ill patients with substance abuse problems engaged in one or more violent acts in the year after they left the hospital. For discharged patients who did not abuse drugs, the corresponding figure was only 18 percent. (That figure suggests that less than one fifth of severely mentally ill individuals without other issues are dangerously aggressive.)

In its meta-analysis, Douglas’s team also flagged drug abuse as one of several factors that contributed to the connection between mental illness and violence. In addition, it found the link was even stronger for patients who suffered from delusions, hallucinations or disorganized thinking. Thus, a mentally ill person is more at risk of committing an act of aggression when that individual is also abusing a drug and shows particular symptoms.